I have a G6DSR.
Within their MediaPlay-Extend media software (which i believe is a hack of Moonshell), the Ebook reader has options for different fonts, font sizes, and vertical/horizontal screen orientation.

I've just ordered an R4, but the standard Moonshell ebook reader doesn't seem to offer vertical orientation. Can anyone tell me how to set this up, or recommend another homebrew reader that does this?

I'm not too sure if I'm on the right track here, but I accidentally opened up a config file and started mashing buttons and it went sideways like in Brain Training/Brain Age... Is that what you're after?